1. Field of the Invention
The present invention relates to a transmitter for sending transmission waves and a receiver for receiving these transmission waves, and particularly relates to a transmitter and transmitting method, receiver and receiving method, pulse position detection method, and tracking method, for ultra wide band (UWB) communication utilizing an impulse signal train at extremely short cycles of several hundred picoseconds to form information signals, and transmit and receives this signal train.
More specifically, the present invention relates to a transmitter and transmitting method, receiver and receiving method, pulse position detection method, and tracking method for transmitting and receiving signals by means of pulses to avoid spectrum problems in an ultra wide band communication system, and relates in particular to a transmitter and transmitting method, receiver and receiving method, pulse position detection method, for ultra wide band (UWB) communication that reduces time required for synchronization with a simple circuit design.
2. Description of Related Art
Local area networks (LAN) configured by connecting multiple computers have become a popular method for sharing information such as electronic files and data or peripheral equipment such as printers and for exchanging information by e-mail and transmitting data contents.
In recent years, wireless LAN have become the focus of attention because wireless LAN eliminates most of the cable wiring used in work spaces such as in offices and makes it relatively easy to move communication terminals such as personal computers (PC). Wireless LAN systems have come into increasing demand as their speed becomes faster and cost becomes less expensive. Very recently, in particular, the use of personal area networks (PAN) made up of small-scale wireless networks for exchanging information among the multiple pieces of electronic equipment used in daily life is under scrutiny
Recently, wireless LAN systems using the SS (Spread Spectrum) method are also being put into practical use. UWB transmission methods utilizing the SS method for applications such as PAN have been disclosed. (Refer to non-patent document 1 cited below as an example.)
The DS (Direct Spread) method, which is one kind of SS method, is also proposed. In the DS method, the transmitter spreads the occupied bandwidth by multiplying an information signal by a random code string called a “PN (Pseudo Noise)” code and transmits the spread information signal, while the receiver performs despreading by multiplying the received spread information signal by the PN code and reproduces the original information signal. In the UWB transmission method, the spread rate of this information signal is maximized to an extreme limit. Data is spread, for example, in an extremely wide bandwidth of 2 GHz to 6 GHz, and transmitted and received to achieve high-speed data transmission.
The UWB transmission method employs an impulse signal train at extremely short cycles of several hundred picoseconds to form information signals, and then transmits and receives this signal train. The occupied bandwidth is on the order of GHz so that the occupied bandwidth divided by its center frequency (for example, 1 GHz to 10 GHz) is approximately 1. This bandwidth is tremendously wide compared to other bandwidths commonly used in the so-called W-CDMA and cdma2000 methods as well as in wireless LAN using the SS (Spread Spectrum) and OFDM (Orthogonal Frequency Division Multiplexing) methods.
Impulse signals used for UWB transmission consist of extremely thin pulses, so the bandwidth used must be extremely wide in terms of frequency spectrum. Because of this requirement, input information signals in each frequency domain have only a very low power which is less than the noise level. Modulation methods for UWB transmission include: PPM (Pulse position Modulation) for expressing a code by means of the position between mono pulses, bi-phase modulation for expressing a code by means of mono pulse phase change, and amplitude modulation.
Non-patent document 1    NIKKEI ELECTRONICS ASIA Mar. 25, 2002    “Ultra Wideband: Revolutionary Wireless Technology is Born”
In the related art, the Gaussian-distributed mono cycle pulse is used as an impulse signal for UWB transmissions. Here, the Gaussian mono cycle pulse and the rectangular waveform mono cycle pulse are compared to determine the linearity required in a pulse generator. AS one example, a rectangular waveform mono cycle pulse with Tp=200 picoseconds [ps] at 1 volt [V] is used. An example with a Gaussian mono cycle pulse is also assumed using the following equation. The constants 3.16 and 3.3 in this equation are values found to possess the same spectrum as the rectangular waveform mono cycle pulse.
                              x          ⁡                      (            t            )                          =                  3.16          ⁢                                          ⁢                      t                          T              P                                ⁢                                          ⁢                      exp            ⁡                          [                                                (                                      3.3                    ⁢                                                                                  ⁢                                          t                                              T                        P                                                                              )                                2                            ]                                                          〈                  Eq          .                                          ⁢          1                〉            
The time waveform is shown in FIG. 1. The frequency characteristics for power spectrum densities of these mono cycle pulses are compared in FIG. 2. However, the power spectrum density [W/Hz=J] is shown when a pulse is sent each second at this voltage and driven at 50 [ohms].
As can be understood in FIG. 2, given a value of 100 [Mpulses per second], a power density which is 80 dB higher than this value is attained. Since the pulse peak power density shown here is approximately −211 [dBJ], this is approximately −41.3 [dBm per MHz] equaling −131.3 [dBW per Hz=dBJ] at 100 [Mpulses per second], which is precisely the boundary specified by the FCC.
The above allows making the following conclusions.
(1) The Gaussian waveform mono cycle pulse and the rectangular waveform mono cycle pulse use nearly the same transmission band.
(2) The mono cycle pulse of the Gaussian waveform requires a higher peak voltage and linearity, and is more difficult to process including power amplification than the rectangular waveform.
Mono cycle pulses are used in conventional UWB communications. FIG. 3 shows the frequency characteristics for power spectrum density shown in FIG. 2, but uses antilogarithm rather than decibels. There is no particular need for using antilogarithm but they often allow a better understanding since energy is shown linearly.
The following 2 points are conditions required on the spectrum.    (1) FCC regulations for spectrum masks do not permit radiation below 3 GHz.    (2) 5 GHz wireless LAN systems are present in the 4.9 to 5.3 GHz band so this band should be avoided.
Examining the power spectrum shown on the linear display reveals the following.
(1) If the above conditions are ignored then transmission will only be about half the power (3 dB).
(2) Ignoring the above conditions will likely cause large distortion on the pulse waveform and only about half the transmission energy will pass through the matching filter on the receive side.
(3) Ignoring the above conditions will cause a total loss of 6 [dB] or more.
The receiver configuration (related art) for the ultra wide band communication system is shown in FIG. 4. The receiver configuration shown in this figure resembles the DS-SS (Direct Sequence Spread Spectrum) receiver configuration.
In the example in the figure, the VCO (voltage-controlled oscillator) oscillates on the same frequency as the pulse period.
The receiver generates a pulse train with data of “All 0” according to the VCO timing. The receiver uses this pulse train to make a total of three waveforms each shifted by half (Tp/2) the pulse width Tp, and multiplies them by the received signal.
To detect the pulse position, a matching pulse timing is found by slightly shifting the VCO frequency (sliding correlation).
When a matching pulse timing is found, the energy increases due to the multiplication results and the pulse position can be detected.
At the stage where the pulse position is detected, operation shifts to tracking simultaneous with returning the intentionally shifted VCO frequency to the correct frequency, in order to maintain the timing.
A positive or negative value corresponding to the positive/negative of the pulse position difference is detected by subtraction of the energy obtained by multiplication with the waveform shifted by ±Tp/2 relative to the puncture (center) is. This value is used as a control voltage for pulse position tracking through the loop filter.
However, when the receiver is configured as shown in FIG. 4, the input signal branches into three paths and must possess three multiplier circuits so the circuit configuration becomes complex.
The frequency must also be changed during search and during tracking. Switching the frequency is time-consuming, so that a long time is required to establish synchronization.
During pulse position detection, the several increases in energy must be detected in order to detect the pulse position correctly in a noisy environment. The pulse position must be detected after first making a slight shift in the frequency, and obtaining the average of energy that increases a number of times, so that a long time is required to establish synchronization.
The device for shifting the frequency and performing tracking is comprised of analog circuits. However, these circuits are complex and affected by fluctuations, making it difficult to achieve stable operation.
During pulse position detection and tracking, the S/N (signal-to-noise) ratio deteriorates and characteristics also deteriorate due to use of the energy value.